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Accueil du site > Divers > equipes de recherche > Matériaux Fonctionnels et de Structure (MFS) > Activités > Métallurgie >

10 février 2011

Introduction

The prediction of defects arising in metallic alloy forming processes and the evaluation of the straining effects in manufactured products are the main objectives of the theoretical and applied research in this thematic. These investigations are supported by obvious economic implications thanks to consequent financial industrial supports.

Among the microstructural rearrangements induced by plastic straining during the forming processes, nucleation and growth of voids and cracks are perhaps the most important7. This leads to unsatisfactory limitations in working operations or unacceptable products. Moreover, an important challenge in this area concerns the soundness of the material undergoing forming operations. For instance, the absence of apparent cracks on the surfaces of the products does not exclude an advanced state of damage within the material volume. Since the internal microstructure of the material is consequently strongly affected by this kind of damage, the mechanical and physical properties of the alloys are also deeply modified. This scientific thematic covers a large area of studies, most of them supported by industrial collaborations. We can cite for example the formability of copper clad aluminium thin wires (with Acome), the damage and fracture of Inconel 601 alloys used in the elaboration of CTN sensors (with Valeo), the interfacial damage and fracture of bainitic steels (Arcelor) or the mechanical behaviour of nitrogen-implanted aluminium alloys (Quertech Ing.). Following are more details on two works devoted to this topic.

 

 

Forming behaviour and size effects

The industrial need of micro metal parts has considerably increased these last years due to the trend of miniaturization of medical, electrical or mechanical devices8. However, the forming process of thin samples is a difficult task due to the low values of thicknesses and / or the low number of grains (diameter d) across the thickness (t). Our works in this area are concerned by the study of the t/d ratio effect on the overall mechanical properties, their correlations with the modifications of the microstructure (grain size distribution, crystallographic texture) and the modifications of the various strengthening mechanisms. Materials studied are Ni and Ni-20wt.%Cr (with general applications concerning the use of microactuators in automotive industry) , Al and Cu (with applications in electronic components forming).

 

Figure 3 – Dislocation structures for a Nickel sample with t/d = 2.5, (a) 250 µm below the free surface and (b) 50 µm below the free surface (Keller et al Mech Mater 2010)

When the t/d ratio decreases, the reduction of the stress level between thicker and thinner samples always stands around 30% (Hug et al, Met Trans A, 2010, to be published). Moreover, this result is verified whatever the strain level and material are. Using the Kocks-Mecking strengthening formalism, these results can be satisfactorily explained by the corresponding evolution of the microstructural parameters governing the dislocation glide. Statistical and quantitative investigations of the dislocation structures were achieved by TEM as a function of strain for core and surface regions. A surface effect systematically appears with a decrease of the intragranular internal stress near the free surface (Keller, Hug, Chateigner, Mat. Sci. Eng. 2009). This decrease is due to the reduction of the interactions between mobile dislocations and those stored in the cell dislocations due to an increase of the mean cell diameter (Fig. 3).

The forming behaviour of nickel sheets can therefore be investigated as a function of the number of grains across the thickness by finite element simulations. Surface effects can be taken into account for the deep drawing simulation of samples by the use of constitutive elasto-plastic laws for surface and core grains (international collaboration with the university of Liege Belgium , the Nanyang Technological University of Singapore and the University of Galati Romania, see Keller et al, Numiform 2010).
 

 

 

 

 

 

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